Oxygen isotope ratio cycle

Oxygen isotope ratio cycles are cyclical variations in the ratio of the abundance of oxygen with an atomic mass of 18 to the abundance of oxygen with an atomic mass of 16 present in some substances, such as polar ice or calcite in ocean core samples, measured with the isotope fractionation. The ratio is linked to water temperature of ancient oceans, which in turn reflects ancient climates. Cycles in the ratio mirror climate changes in geologic history.

Oxygen (chemical symbol O) has three naturally occurring isotopes: ¹⁶O, ¹⁷O, and ¹⁸O, where the 16, 17 and 18 refer to the atomic mass. The most abundant is ¹⁶O, with a small percentage of ¹⁸O and an even smaller percentage of ¹⁷O. Oxygen isotope analysis considers only the ratio of ¹⁸O to ¹⁶O present in a sample.

The calculated ratio of the masses of each present in the sample is then compared to a standard, which can yield information about the temperature at which the sample was formed - see Proxy (climate) for details.

¹⁸O is two neutrons heavier than ¹⁶O and causes the water molecule in which it occurs to be heavier by that amount. The addition of more energy is required to vaporize H₂¹⁸O than H₂¹⁶O, and H₂¹⁸O liberates more energy when it condenses. In addition, H₂¹⁶O tends to diffuse more rapidly.

Because H₂¹⁶O requires less energy to vaporize, and is more likely to diffuse to the liquid surface, the first water vapor formed during evaporation of liquid water is enriched in H₂¹⁶O, and the residual liquid is enriched in H₂¹⁸O. When water vapor condenses into liquid, H₂¹⁸O preferentially enters the liquid, while H₂¹⁶O is concentrated in the remaining vapor.

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